The invention relates to a technique and device for increasing the number of colors output by a passive liquid crystal display.
Passive color liquid crystal (LCD) displays are commonly known as STN or DSTN panel displays, and are commonly used in laptop computers to present information to a user. Passive color LCD panels are similar to color cathode-ray tube (CRT) monitors in that the resolution depends upon the number of pixels in the display. Typical resolutions are 640 columns of pixels by 480 rows of pixels (640xc3x97480), 800xc3x97600 pixels and 1024xc3x97768 pixels. However, the CRT monitor uses analog data to form images on its screen, while a passive color LCD panel displays uses digital data.
Each pixel consists of three primary color components: red, green and blue. Consequently, passive color LCD displays are capable of displaying eight colors: white, black, red, green, blue, magenta, cyan and yellow. The colors are created by generating digital data that controls the ON and OFF state of each color component of the pixels on the screen. For example, if a digital 111 signal is supplied to every pixel then all three color components for each pixel are ON, and the display screen will appear white in color to the human eye. However, a digital 000 signal supplied to every pixel turns all of the color components OFF and the display screen appears black in color. If a digital 100 signal is supplied to all pixels, then the red components are ON and the blue and green components are OFF so that the screen appears red in color. Thus, by controlling the combinations of color components which are ON and OFF for each pixel, the eight colors identified above can be generated.
New data must be supplied to the pixels of a passive color LCD display periodically to refresh the image shown on the screen, and such time segments are known as frames. Three bits of data per pixel is typically supplied every {fraction (1/60)}th of a second, which corresponds to a refresh rate of sixty frames per second.
Since each color component in the above example can only be turned ON or OFF, the grey level for each color component is two. In order to increase the number of grey levels and thus the number of colors that can be displayed, some prior art passive color LCD panels use a technique known as frame modulation. Using such a technique, some manufacturers claim that their passive LCD displays are capable of displaying as many as 256 colors. However, there is a need for not only increasing the number of colors that may be displayed by a passive color LCD display, but also for improving the overall quality of the color and for minimizing any flicker of the screen which can be detected by the human eye.
The invention increases the number of colors output by a passive liquid crystal display by providing an increased number of grey levels.
In general, the invention features generating a Mxc3x97N matrix pattern of pixel components on the display having a ratio of pixel components that are ON to the total number of pixel components to achieve a particular grey level, wherein M and N are greater or equal to two. The Mxc3x97N matrix pattern is produced for X frames, wherein at least one pixel component is ON in each frame.
Preferred embodiments include the following features. In the Mxc3x97N matrix pattern the same number of pixel components are ON in each frame but in different locations. In addition, at least one of the pixel components is ON in each row and column in each frame. Further, over the course of X frames, each pixel component is turned ON for xe2x80x9cYxe2x80x9d amount of times, wherein xe2x80x9cYxe2x80x9d equals the number of pixel components in each row or column that is ON in any one frame. Yet further, the M, N and X variables are all equal, so that a square dimension matrix is generated which is repeated for the same number of frames as the dimension. Additionally, the value of X is chosen so that the number of frame cycles of a particular grey level matrix pattern is not a multiple of the frame cycle of another grey level matrix pattern. A plurality of grey level matrix patterns are generated having an average brightness that varies over the full range of a pixel component, and preferably 16 grey level matrix patterns are utilized. The average brightness level of at least some of the 16 grey level matrix patterns is not an increment of 16.
In another aspect of the invention, preferred embodiments include square-dimension matrix patterns to produce sixteen grey levels. In particular, 17xc3x9717 matrix patterns are described having two pixel components ON in each row or column to generate grey level 1, twelve pixel components ON in each row or column to generate grey level 11, and fifteen pixel components ON to generate grey level 14. Also, 5xc3x975 matrix patterns are described having one pixel component ON in each row or column to generate grey level 2, two pixel components ON in each row or column to generate grey level 5, three pixel components ON in each row or column to generate grey level 9, and four pixel components ON in each row and column to generate grey level 13. Two 4xc3x974 matrix patterns are described having one pixel component ON in each row and column to generate grey level 3, and three pixel components ON in each row or column to generate grey level 12. Similarly, two 3xc3x973 matrix patterns are disclosed having one and two pixel components ON in each row and column, to generate grey levels 4 and 10, respectively. A 2xc3x972 matrix pattern with one pixel component ON in each row is used to generate grey level 7, and two 7xc3x977 matrix patterns are described having three and four pixel components ON to generate grey level 6 and grey level 8, respectively.
In a further aspect of the invention, a grey level generator to produce the matrix patterns according to the invention is described. In particular, a memory is provided for storing Mxc3x97N matrix data, in addition to a frame counter for counting to X frames, a column counter for counting to N, and a row counter for counting to M. The row counter is pre-loaded with a value for a pixel component at the beginning of each frame based on the data stored in memory. A comparator generates an output signal to the passive LCD display indicating which pixel components should be ON or OFF depending on the frame and their row and column location.